Thumbnail problem on multisite

My new installed multisite system for two-languages site (subfolders, SnapWire theme) has a problem with thumbnails and images for top dynamic slider. I can see thumbnails files available in folders but I can’t see them on the site. The only pictures I see are the illustrations in open articles.

I use one theme for both sites (with different languages). I also changed the theme just for test – still no thumbnails.

Can anybody give me a clue – where to go? Thank you.